Is there a way to stop Cycles Computing Transparency?
From my knowledge of cycles, I don't think this is possible.

Think of it this way:  If you disable camera ray visibility it simply ignores the object when calculating ray collisions.  But a transparent shader is simply that: a shader.  And Cycles will only look at a surface's shader if a ray has already hit the surface, meaning the calculation you are trying to avoid has already been performed.  In other words, Cycles has to perform the collision algorithm to find out where the ray hits the surface to know if it is transparent.

To ignore transparent parts of objects you would first have to go through every shader in the scene looking for the pure white 100% transparent parts so that it just so you can ignore them in the ray-tracing step.  This process would be very computationally expensive and would probably far out-weigh the saved time in the ray collision calculation.

With that said, I am by no means a complete expert on cycles, so if anything I have said is incorrect please let me know.